In order to realize higher-density recording which results in higher-density reproduction, it is important to reduce the grain size of the recording layer of magnetic media. Several approaches, such as reducing the thickness of a crystalline seed layer and/or a Ru/Ru alloy interlayer, or alloying the crystalline seed layer, have been proposed to try to reduce a grain size of the interlayer and recording layer above the interlayer. These approaches, however, bring undesirable consequences, such as size distribution widening, degradation in crystallographic texture (particularly in the C-axis orientation), degradation in magnetic decoupling in the recording layer, etc. Therefore, enlargement of the magnetic cluster size occurs in many cases. The problem here is that physically smaller grain size is not necessarily equivalent to magnetically smaller cluster sizes. This undesirable nature makes it difficult to improve recording and reproduction characteristics of magnetic heads and magnetic media, and in most cases, recording and reproduction characteristics become worse because of the physically small grain size of the recording layer.
Presently, the areal recording density of magnetic disk devices, such as hard disk drives (HDDs), is at least several hundreds of Gbit/in2, and as the areal recording density increases, recording methods have moved away from the longitudinal magnetic recording method to the perpendicular recording method. When high-density recording is carried out with such a method, magnetic flux leakage from adjacent bits acts in the direction which stabilizes the magnetization, and therefore this method is clearly more effective for increasing density than longitudinal magnetic recording. In order to achieve higher density recording in the future, such as on the order of 1 Tbit/in2, the bits will need to be finer, and it will be necessary to refine the grain size in the recording film accordingly.
Investigations have been carried out in the past into refining the recording film grain size and promoting crystal growth using techniques involving thinner seed layers and seed layers made of various kinds of materials, which are discussed below.
U.S. Patent Appl. No. 2010/0021770 discloses that crystal growth is enhanced when a Ni alloy is used in the seed layer in a perpendicular magnetic recording medium. Japanese Patent Pub. No. JP 2009-245484 discloses that it is possible to refine the size of the magnetic grains without reducing the magnetic grain density in magnetic layers when Ni is the main component in a perpendicular magnetic recording medium and use is made of an orientation control layer to which an oxide has been added. However, the orientation control layer in this medium is a single layer, and the rate of refinement is less than 10%. U.S. Patent Appl. No. 2007/0231609 discloses that crystal growth in the interlayers is promoted by using an underlayer containing Al, Cu, Ni, Pt, and NiFe in the perpendicular magnetic recording medium.
However, even though crystal growth is promoted in these methods, there are problems in that the crystal grains are not refined or the refining effect is small, or in that there is a deterioration in the C-axis crystal orientation (orientation in the perpendicular direction) when the recording film grain size is sufficiently small, which impairs the recording and playback characteristics.
Therefore, it would be beneficial to have a magnetic recording medium and recording/playback apparatus that could avoid the limitations and problems of conventional devices.